1. Field of the Invention
The present invention relates to a rare earth permanent magnet exhibiting excellent magnetic properties such as coercive force, and improved electric and electronic equipment in which the magnet is used.
2. Description of the Prior Art
Sm,Co-containing magnets are among the most commonly used high performance rare earth permanent magnets used in equipment, such as, loud speakers, motors, and various measuring instruments. However, samarium and cobalt are relatively expensive, and when used as raw materials in mass production, are the chief barrier to attaining economical production. To improve the economy of the process, as well as to upgrade the magnetic properties of the product magnets, the samarium content is reduced and the cobalt is replaced as much as possible by iron.
The conventional SmCo.sub.5 type permanent magnets are based on a SmCo.sub.5 compound having the hexagonal CaCu.sub.5 structure (hereinbelow referred to as "the 1/5 structure" or "the 1/5 phase). Since these magnets are crystallographically balanced, it is impossible to reduce the Sm content and it is impossible to replace a part of cobalt with iron.
The conventional Sm.sub.2 Co.sub.17 type permanent magnets are based on a Sm.sub.2 Co.sub.17 compound having the rhombohedral Th.sub.2 Zn.sub.17 structure (hereinbelow referred to as "the 2/17 structure" or "the phase 2/17 phase"). The Sm content of the Sm.sub.2 Co.sub.17 type permanent magnet is about 8% lower than that of the SmCo.sub.5 type permanent magnet. Also, while desired, no more than 20 at. % of the cobalt in the Sm.sub.2 Co.sub.17 type permanent magnet can be replaced by iron without affecting the magnetic properties [T. Ojima et al, LEEE Trans Mag Mag-13, (1077) 1317]. In order to give rise to two phases in the Sm.sub.2 Co.sub.17 type permanent magnet, inclusion of copper is essential. However, since Cu is a non-magnetic element, the amount of Cu should be as small as possible. For example, in a conventional magnetic compound of the formula Sm(CoFeCuM).sub.z , the molar fraction of Cu based on the non-samarium elements can be reduced, at best, to 0.05. Further reduction leads to a precipitous decrease in intrinsic coercive force (iHc) [Tawara et al, Japanese Applied Magnetics Symposium 9, ( 1985) 20].
In the conventional Sm.sub.2 Co.sub.17 type permanent magnets which are sintered in the manufacturing process, the molar ratio of Sm to non-samarium elements is often 1/7.5, i.e. z=7.5. However, in Sm.sub.2 Co.sub.17 type permanent magnets, e.g., plastic magnets, which are directly heat-treated while in the ingot form rather than made by means of the powder sintering method and therefore not sintered, the usual molar ratio of Sm to non-samarium elements is from 1/8.0 to 1/8.2 [T. Shimoda, 4th International Workshop on Re-Co Permanent Magnets p.335 (1979)].
The binary-phase separation in the 2/17 magnets generally occurs such that the resulting phases are of SmCo.sub.5 and Sm.sub.2 Co.sub.17 compounds respectively, so that theoretically the molar ratio of Sm to non-samarium elements cannot be smaller than 1/8.5.
The above-referenced thesis of T. Shimoda discloses an example wherein the molar ratio of Sm to non-samarium was 1/8.94. However, since Sm.sub.2 Co.sub.17 and Co coexist in the magnet of this example, the squareness of the magnetic hysteresis loop is substantially lost, i.e., the value given by 4Br.sup.-2 (BH).sub.max becomes far smaller than unity, wherein Br is the residual magnetization. Consequently the magnet of the example cannot be put to practical use.
Attempts to reduce the contents of Sm and Cu and to increase the Fe content in the samarium cobalt magnets have not been successful.
Nagel reported on a nucleation growth-type samarium magnet which contains no copper [H. Nage, 3M Confererence Proc. 29 (1976) 603]. However, this magnet has not been put to practical use because its coercive force undergoes wide changes with temperature.
The recently developed Nd-Fe-B magnets have higher magnetic properties than Sm-Co magnets, and are advantageous since they mainly comprise readily available. However, since neodymium has a high tendency to oxidize, it is necessary to hermetically coat the magnets containing Nd to prevent rusting. This necessity of coating, as well as the difficulty in finding appropriate coating materials suitable for mass production of Nd-Fe-B magnets, has thwarted economical mass production of the magnets.
The residual magnetization (Br) and the intrinsic coercive force (iHc) of the Nd-Fe-B magnets decreased sharply as the temperature rises, which is extremely inconvenient in practical use. Consequently, the operational temperature ranges of the Nd-Fe-B magnets are severely restricted especially due to the thermal instability of the intrinsic coercive force [D. Li, J. Appl. Phys 57(1985)4140]. The poor stability of the intrinsic coercive force is ascribable to the fact that the coercive force of the Nd-Fe-B magnets are given rise to by the nucleation growth of the crystal. As is the case with the Sm magnet of Nagel, it is, in principle, impossible to reduce the temperature coefficient of the intrinsic coercive force of the Nd-Fe-B magnets. The temperature coefficient of the intrinsic coercive force iHc of the Sm-Co magnets, whose coercive force results from the binary-phase structure, is less than that of the Nd magnets whose coercive force results from the nucleation growth of the crystal. Therefore, the Sm-Co magnets are more reliable in applications where high temperatures are encountered.
Previously we invented two kinds of rare earth magnets wherein the main phases are, respectively, of the RFe.sub.12-x M.sub.x composition having the body-centered tetragonal lattice 1/12 structure (ThMn.sub.12 structure) and of the R(Fe.sub.1-x Co.sub.x).sub.12-y M.sub.y composition (Japanese Patent Applications Nos. 62-224764 and 62-233481).